Thought this was a pretty interesting way to calculate risk. Although it is based on their own internal risk assessments, it might make for a good starting point in your own organization when talking to higher ups or generating a report to a client:

This calculator creates a CVSSv2 base score for vulnerabilities in web applications based on the High-Tech Bridge internal scoring system that is implemented in our HTB Security Advisories and is used to calculate risk of discovered vulnerabilities.

Not all vulnerabilities are scored in strict accordance to FIRST recommendations. Our CVSSv2 scores are based on our long internal experience in web applications auditing and penetration testing, taking into consideration a lot of practical nuances and details. Therefore sometimes they may differ from those ones that are recommended by FIRST.